The present invention relates to an inexpensive method and apparatus for sensing the height of a stack of media sheets, such as paper, used by media-using-machines such as desk top printers, fax machines, and the like, that are sheet fed from a media holder such as an input hopper or tray.
There are a variety of media-using-machines that are fed from a stack of sheets of media. For example, desktop printers, fax machines, and text scanning machines typically operate in this fashion with the media being paper, plastic for transparencies and so forth. The media stack is held in input hoppers or trays which may be removable or built-in.
FIG. 1 is a perspective view which shows a prior art sheet fed media-using-machine 20. Machine 20 has a media holder 22 and is fed with media sheets 24 which are held in media holder 22. In FIG. 1, media holder 22 is illustrated as a tray but other embodiments, such as a slot into which a stack of media sheets 24 are directly placed may also be employed. Media holder 22 is inserted into holder slot 26 and is thus housed within or attached to machine 20. Media holder 22 is illustrated as a removable tray in which media sheets 24 form a media stack. Media sheets 24 move in the direction of arrow 22A and are fed into media-using-machine 20. Media sheets are withdrawn sheet-at-a-time (sheetwise) from what is referred to as the top of a stack. Media sheets 24 are processed within machine 20, such as by printing upon the media or optically scanning the media, and are ejected from media eject opening 28 in the direction of arrow 28A to be deposited into the pickup hopper location of sheet 24A. Machine 20 has within it a control system 29 for controlling the operations of machine 20 that are described in this paragraph.
In prior art desktop printers, if the media holder 22 becomes empty such as resulting from normal use of all media sheets 24 that are held in media holder 22, then control system 29 may cause an audible or visible signal (not shown) on machine 20 to activate. An operator using machine 20 may notice the alert signal, realize that no media sheets 24 remain in media holder 22 and then refill media holder 22 with additional media sheets 24. Alternatively, or in addition to the foregoing signal, if in the process of printing a print job the printer is depleted of paper, then control system 29 may cause a message to appear on a display, such as a message to the effect xe2x80x9cthe printer is out of paper, do you wish to continue ( ) yes, or ( ) no?xe2x80x9d This message is accompanied by a pause in machine operation since no media sheets 24 remain in media holder 22. Once the operator has refilled media holder 22 and replaced it in holder slot 26, the operator may click a computer mouse in the xe2x80x9cyesxe2x80x9d location and proceed with the print job. Alternatively, the operator may cancel the job. Alternatively, the machine 20 may automatically sense that the media holder 22 is no longer empty and begin to continue a print job.
FIG. 2 is a cut-away view which provides further details of the prior art media holder 22. In this and in other figures described in this specification, like numerals are identified with like reference numbers. Media holder 22 provides a structure for supporting media sheets 24, with the structure comprised of tray bottom 30A which provides a common supporting structure for the sides of media holder 22. These sides are front side 30B, right side 30C, back side 30D, and left side 30E (not shown). Media sheets 24 are stacked into media holder 22 with tray bottom 30A providing vertical support to the sheets and with sides 30B, 30C, 30D, and 30E providing a structure to retain media sheets 24 stacked in a vertical column of sheets, one atop the another. The front side 30B is xe2x80x9cfrontxe2x80x9d with reference to flow of media sheets 24 from media holder 22, that is, as media sheets 24 are fed from media stack 24 into the media processing areas of machine 20, such media sheets 24 flow past front side 30B. Right side 30C and left side 30E are right and left, respectively, with reference to view in the direction of media feed direction, illustrated by arrow 22A. The back side 30D is the side that is opposite the front side.
Stack pusher 32 is disposed to provide a mechanical pushing force to push media sheets 24 (the media stack) into a position such that machine 20 may withdraw the sheets from the stack. Stack pusher 32 consists of pusher plate 32A, pusher base 32B, hinge 32C, and spring 32D. Pusher plate 32A is pivotally connected by hinge 32C to pusher base 32B. Spring 32D provides a compressive force to urge pusher plate 32A away from pusher base 32B. Pusher base 32B may be attached to tray bottom 30A, or tray bottom 30 may serve as pusher base 32B. This urging force of spring 32D causes pusher plate 32A to push media sheets 24 into a position that machine 20 may withdraw the sheets from the stack. It should be noted that while the illustration shows spring 32D pushing pusher plate 32A, the prior art includes springs disposed to exert tensile force pull to pusher plate 32A, and the prior art includes rotational forces to torque pusher plate 32A, all to provide the same result of pushing media stack 24.
FIG. 3 is a perspective view of the prior art media holder 22 of FIG. 2. In this view, right side 30C, back side 30D, and left side 30E are attached to tray bottom 30A. Window 30F is provided on back side 30D. Window 30F is a vacancy or an aperture which is provided to allow a user of machine 20 to determine the presence or absence of media sheets 24 in media holder 22, or to estimate the number of media sheets 24 in media holder 22, without the need for withdrawing media holder 22 from holder slot 26. Thus window 30F provides a convenient way of ascertaining the amount of media sheets 24 in media holder 22. A limitation of window 30F is that it does not provide quantity information to control system 29. Another limitation of window 30F is that it requires the active attention of the operator of machine 20, that is, the operator must pay attention to window 30F. Yet another limitation is that machine 20 must be positioned or oriented on a desk or table such that the window is easily within view. In practice, when media sheets 24 are fully depleted, machine 20 stops operation. The view provided by window 30F becomes simply a confirmation of what the operator already expects when machine 20 stops operation, that is, confirmation that media holder 22 is indeed empty with the convenience being that confirmation can be performed without withdrawing media holder 22 from machine 20 for inspection.
FIG. 4 is a perspective view of a prior art media holder 22 with some differences to that illustrated by FIG. 3. In FIG. 4, pusher plate 32A has tabs 36A which project into slots 36B. Slots 36B are indentations, grooves, or the like, that are built into right side 30C and left side 30E. Tabs 36A and slots 36B allow pusher plate 32A to change position in a constrained translational motion fashion in contrast to the rotational motion featured by prior art media holders 22 of FIGS. 2 and 3 having a hinge 32C allowing constrained rotational movement of pusher plate 32A.
FIG. 4 also illustrates indicator mechanism 38. Indicator mechanism 38 is a mechanism which is provided to allow a user of machine 20 to determine the presence or absence of media sheets 24 in media holder 22, or to estimate the number of media sheets 24 in media holder 22, without the need for withdrawing media holder 22 from holder slot 26. While indicator window 30F provided a direct viewing means, indicator mechanism 38 provides an indirect way of accomplishing the same result. Indicator mechanism 38 is comprised of indicator lever 38A having indicator end 38B which projects into window 30F. Indicator lever 38A has an opposite end, that is, a paper contact end 38C. Indicator pivot 38D is a pivot point that is supported by media holder 22, such as by side 30E or by holder bottom 30A. Indicator lever 38A is pivotally mounted on indicator pivot 38D Double arrow 38E indicates the two directions that paper contact end 38C can travel. As additional sheets of media are properly placed into media holder 22 with sheets positioned under paper contact end 38C, paper contact end 38C is pushed upward and the pivot/lever action causes indicator end 38B to move downward. The relatively downward positioned indicator end 38B shows the machine user that media holder 22 is relatively full of media sheets 24. Alternatively, indicator mechanism 38 can be located such that media sheets push paper contact end 38C downward. In this case, the indicator end 38B remains up until it falls downward as the last sheet is removed, i.e., only when the tray 22 becomes empty. In the examples of indicator mechanism just described, a spring exerting tensile or compressive force may be employed to bias indicator lever 38A towards a selected position with a counter-force being exerted by the stack of media sheets.
Unfortunately, the indicator mechanism 38 of FIG. 4 suffers from the same limitations as those of window 30F that were described in connection with window 30F of FIG. 3.
FIG. 5 is a cut-away view which provides further details of the prior art media holder 22 of FIG. 4. Note the tabs are identified with reference numeral 36A. It should also be noted that while spring 32D is illustrated as a compressed spring urging pusher plate 32A away from bottom 30A, an alternative spring is sometimes used in the prior art, and that alternative spring is a spring in tension, pulling pusher plate away from bottom 30A to accomplish the same result.
FIG. 6 illustrates an aspect of prior art media holders 22 in that media feed rollers 39 are illustrated. Media feed rollers 39 are part of the media feed mechanism of media-using-machine 20 and which are used to withdraw sheets from a stack of media sheets 24. Note that the stack pusher 32 is illustrated with pusher plate 32A relatively close to pusher base 32B. This corresponds to a media holder 22 that is relatively full of media sheets 24.
FIG. 7 illustrates an aspect of prior art media holders 22 similar to that illustrated by FIG. 6. In FIG. 7, stack pusher 32 is illustrated with pusher plate 32A relatively away from pusher base 32B. This corresponds to a media holder 22 that not relatively full of media sheets.
FIGS. 6 and 7 illustrate a media stack 24 in a rest position (FIG. 6) and in a lifted or pushed position (FIG. 7). No spring is shown for media pusher 32 of FIG. 7, and in this media pusher 32 the media stack 24 pushing force is provided by the rotation of a bar (not shown) located on tray bottom 30A and hinged about one of the edges of pusher plate 32A. The lifting bar rotates the pusher plate 32A upward until the media stack 24 is pressed against media feed rollers 39 of the media feed mechanism. The angle to which the bar rotates is therefore an indication of the quantity of sheets in media stack 24.
From the limitations in the prior art that are described above, it can be seen that it is desirable to improve the ways in which the user of a sheet fed machine can determine the quantity of sheets in the media holder of such a machine. Also, since there is constant effort by manufacturers of such machines to make machines that are inexpensive, it can be understood that provision of low cost ways to determine the quantity of sheets is a highly desired goal.
The present invention employs an inexpensive method and apparatus for sensing the height of a stack of media. A sheet fed media-using-machine has a media holder. The media holder holds a stack of a variable quantity of sheets of a medium to be supplied to a media-using-machine. The media holder has a structure for supporting the sheets and has a stack pusher which may be a mechanical device that pushes the stack into a position such that said media-using-machine may withdraw the sheets from said stack. The stack pusher has a variable position of displacement within a range of motion wherein such position is dependent upon a quantity of sheets in the stack. A variable device is attached to the stack pusher such that an electrical property, or other property that can be sensed of said variable device, varies in relation to the position thereby allowing determination of an estimate of the quantity of sheets in said stack, where the determination is performed using the electrical or other property. The variable device may be, for example, a capacitor or a resistor, and the electrical property being capacitance or resistance, respectively.